The present invention is directed to an apparatus for measuring projectile velocity and, in particular, to an apparatus for measuring the velocity of truncated stern projectiles by means of a light screen, a photo sensor, and a signal processor.
Light screens and in particular laser light screens for ballistic measurements are known to the art. See, e.g., West German DT-OS No. 26 10 708. In such arrangements of the prior art, a light screen is interrupted at the passage of a projectile. A photo sensor is used to monitor the light screen, and the modification of the photo sensor signal associated with the passage of a projectile through the light screen is used to start and stop a time counter. The time difference between the entry of the projectile into a first light screen (which triggers a start signal for the time counter) and the entry of the projectile into a second light screen (which triggers a stop signal for the time counter) is used to determine the projectile's speed. The photo sensor signal associated with the passage of a projectile through a light screen has a curve which is a function of the geometry of the projectile. Here, the trace of the curve corresponds to the signal obtained from the photo sensor as a function of time.
A precise measure of the instant of passage of the projectile through the light screen cannot be determined merely by triggering the time counter at an undetermined point during the deviation of the photo sensor signal. Because of the length of the projectile, a specific amount of time elapses between entry of the projectile into the light screen and its exit therefrom. Within the period of this passage time, the instant of passage is thus undefined. As mentioned, a reproduceable measure of the instant of passage is needed to precisely start and stop the time counter.